Why Is Welding A High-Risk Stage For HIC?

Hydrogen-induced cracking (HIC) poses a severe threat to the structural integrity of LSAW (Longitudinal Submerged Arc Welded) steel pipes, which are crucial for high-pressure energy transmission and heavy infrastructure. While raw material quality lays the foundation for HIC susceptibility, the welding stage is widely recognized as the highest-risk period for HIC initiation. This is because welding involves complex thermal and chemical processes that can lead to massive hydrogen ingress, residual stress accumulation, and microstructural changes-all of which synergistically trigger HIC. Hebei Huayang Steel Pipe, a leading LSAW manufacturer with 22 years of experience and a CNAS-certified laboratory, has accumulated abundant data through thousands of welding tests and on-site verifications. This article explores why welding becomes a high-risk stage for HIC, shedding light on key risk factors and industry-wide prevention strategies.

Hydrogen is the core driver of HIC, and the welding process is the primary source of hydrogen ingress into LSAW pipes-even more significant than hydrogen in raw materials. The high-temperature welding arc creates an environment where various hydrogen-containing substances decompose, releasing hydrogen atoms that are easily absorbed by the molten weld metal.

Huayang's laboratory tests reveal three main pathways for welding-induced hydrogen: damp welding materials, surface contaminants, and atmospheric moisture. Welding flux and wire, if not properly dried, contain moisture that decomposes into hydrogen under arc heat. Huayang's technical standards mandate that HJ431 flux be dried at 300℃ for 2 hours to ensure moisture content ≤ 0.1%. A production incident in 2023 proved the danger of non-compliance: when operators skipped the drying step, weld hydrogen content surged from the standard 4ppm to 8ppm, resulting in 3 out of 10 test blocks developing HIC cracks. Additionally, oil, rust, and paint on steel plate edges react with the arc to generate hydrogen. Huayang's pre-welding cleaning protocol-requiring edge grinding to remove contaminants within 20mm of the weld zone-reduces such hydrogen ingress by 30%.

Welding parameters, such as current, voltage, and speed, directly influence the cooling rate of the weld and the efficiency of hydrogen escape. Improper parameter settings can trap hydrogen in the weld metal and heat-affected zone (HAZ), significantly elevating HIC risks.

Huayang's comparative tests on DN800 LSAW pipes demonstrated this correlation. When welding speed increased from 0.3m/min to 0.5m/min, the weld cooling rate accelerated by 30%, leaving insufficient time for hydrogen to diffuse out. This caused hydrogen content to rise from 4ppm to 6.2ppm, doubling the HIC cracking rate. Excessively high current (over 650A) also proved problematic: it intensified welding material burnout, increased arc instability, and promoted hydrogen absorption. To address this, Huayang equipped all LSAW lines with digital monitoring systems that track 16 key parameters in real time. The system alerts operators to deviations, ensuring parameters remain within the optimal range for hydrogen escape-current 550-600A, voltage 28-32V, and speed 0.35-0.45m/min.

Welding generates substantial residual stress in the weld and HAZ, which acts as a "catalyst" for HIC. This stress creates "hydrogen traps" that hinder hydrogen diffusion and concentrate hydrogen at microdefects, driving crack initiation and propagation.

The thermal cycling of welding-rapid heating to 1500℃+ followed by rapid cooling-causes uneven expansion and contraction of the steel, resulting in tensile residual stress of up to 300MPa in the weld zone. Huayang's stress analysis tests show that this residual stress, combined with hydrogen enrichment (≥5ppm), can reduce the critical hydrogen concentration for HIC by 40%. For high-pressure LSAW pipes, Huayang mandates post-weld stress relief annealing at 620℃ for 2 hours, which releases 80% of residual stress and reduces weld hydrogen content to below 2ppm. A 2024 project for deep-sea oilfields confirmed the effectiveness of this process: annealed pipes showed zero HIC cracks in subsequent tests, compared to a 6% cracking rate in unannealed samples.

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The heat-affected zone (HAZ)-the area adjacent to the weld that is heated but not melted-emerges as the most HIC-susceptible region during welding. Its microstructural changes and hydrogen sensitivity make it a weak link in LSAW pipes.

Huayang's metallographic analysis reveals that the HAZ undergoes drastic microstructural transformations: the coarse-grained region (CGHAZ) forms large austenite grains during welding, which transform into brittle martensite and bainite during rapid cooling. These brittle phases have low toughness and high hydrogen solubility, making them prone to hydrogen aggregation. Tests show that the CGHAZ has a hydrogen solubility 1.5 times higher than the base metal and a -20℃ impact toughness 30% lower. To mitigate this, Huayang optimizes the welding heat input (controlled at 25-35kJ/cm) to refine HAZ grains and reduce brittle phase formation. Additionally, the company uses low-hydrogen welding materials to minimize hydrogen absorption in the HAZ, further lowering HIC risks.

Hebei Huayang Steel Pipe's long-term practice has yielded a comprehensive welding quality control system that effectively reduces HIC risks. This system focuses on four core aspects: low-hydrogen material management, precise parameter control, strict protection measures, and mandatory post-weld treatment.

Huayang uses only low-hydrogen welding wires (H08MnMoA) and fluxes, with intelligent drying ovens to monitor moisture content. Digital welding monitoring systems ensure parameter stability, while automated flux feeding devices maintain a flux coverage thickness of ≥20mm to prevent arc exposure. Pre-welding edge cleaning and post-welding stress relief annealing are strictly enforced. For the 2023 Central Asian oil and gas pipeline project, these measures reduced the HIC cracking rate of welded LSAW pipes to 0.2%, far below the industry average of 1.2%.

Welding becomes a high-risk stage for HIC due to the combined effects of excessive hydrogen ingress, improper parameter settings leading to hydrogen trapping, residual stress accumulation, and HAZ microstructural brittleness. These factors create a "perfect storm" for HIC initiation and propagation. Hebei Huayang Steel Pipe's experience demonstrates that through systematic control of welding materials, parameters, protection, and post-weld treatment, HIC risks can be effectively mitigated.

As LSAW pipes are increasingly used in harsh environments such as high pressure, deep sea, and corrosive conditions, strict welding quality control becomes even more critical. By understanding and addressing the root causes of welding-induced HIC, manufacturers can enhance pipe reliability and ensure the safe operation of energy and infrastructure projects worldwide.